The quasilinear large-amplitude viscoelastic regime and its significance in the rheological characterization of soft matter

We present a novel approach for using large-amplitude oscillatory shear flow experiments (LAOS) to determine—in a simple, direct, and robust manner—the mechanical behavior of materials that possess a microstructure and may exhibit solidlike, liquidlike, viscoelastic, viscoplastic, and/or thixotropic behavior. As the stress amplitude and frequency are independently varied, two classes of motion are observed: (i) structure-changing motions, characterized by a nonsinusoidal response, when the stress amplitude is large enough to cause microstructural changes and the frequency is of the order of the reciprocal of the time scale of microstructural changes; and (ii) constant-structure motions, characterized by a sinusoidal response, when either the stress amplitude is not large enough to cause microstructural changes or the stress amplitude is large enough to cause microstructural changes but the frequency is much larger than the reciprocal of the time scale of microstructural changes. For a commercial hair gel, we confirm experimentally the existence of these two classes of motions, which was theoretically predicted by de Souza Mendes and Thompson [Rheol. Acta 52(7), 673–694 (2013)]. In contrast to the presently available LAOS analyses, which focus on the structure-changing motions, our methodology—quasilinear LAOS or QL-LAOS—relies for the most part on data obtained from constant-structure motions. This fact simplifies dramatically the experiments and the analysis and provides material functions the physical meanings of which are quite evident.